1. Field of the Invention
The present invention relates generally to field emission cathodes for x-ray radiation sources. More particularly, the present invention relates to carbon nanotube field emission cathodes and methods of manufacture and operation of such cathodes in linear or area x-ray radiation sources with individually addressable multi-beam x-rays suitable for use in diagnostic, imaging, and inspection applications.
2. Background of the Invention
In the description of the background of the present invention that follows reference is made to certain structures and methods. Such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Computed tomography (CT) technology is widely used for medical, industrial and security imaging purposes. The designs of typical computed tomography machines have gone through major evolutions. For example, for conventional x-ray imaging, a three-dimensional (3-D) object is illuminated to form a two-dimensional (2-D) image. As a result, the spatial resolution in the illumination direction is lost. This limitation can be overcome in computed tomography systems by obtaining projection images of the object in different directions. Typically, the object is stationary while a single x-ray source rotates around the object and produces the images at different rotation angles. The collection of the projected images can then be used to reconstruct a three-dimensional image of the object.
Rotation of the x-ray source puts considerable demand on the system design and can reduce the imaging speed. An electron-beam computed tomography (EBCT) system can address this problem. In typical EBCT systems, electrons produced by the cathode are scanned across the surface of the anode located in the gantry, which consists of a metal ring or multiple rings. The scanning is accomplished by electrical and magnetic fields. However, the machine is expensive and takes significantly larger space than a regular computed tomography system. Thus, it is highly desirable to have a small stationary x-ray source computed tomography system that is potentially more transportable and cost effective.
In some systems, such as tomography, the x-ray source is stationary and the object is rotated to collect the projection images. In the micro-computed tomography systems, the x-ray source typically produces a fan beam onto the object. In some cases, a cone beam and two-dimensional detector are used to record the images. The object is rotated and an image is collected at every rotation angle. An example of the two-dimensional area detector consists of a scintillation crystal that converts the x-ray photon to visible light, and a charge-coupled-detector (CCD) camera positioned behind the crystal that captures the image. Solid state and gas detectors are also commonly used.
From the point of view of image quality, it is preferred to use a monochromatic x-ray. This is because computed tomography measures, essentially, the linear absorption coefficient, which depends on the energy of the incident x-ray photon. However, in most computed tomography systems, with the exception of a synchrotron radiation source, continuous-energy x-ray rather than monochromatic x-ray is used so as to increase the x-ray intensity, and thus reduce the data collection time. In many computed tomography systems, the x-ray source is often placed far away from the object to minimize the non-even spatial distribution of the x-ray radiation from the single x-ray source and the divergence of the x-ray beam. As a result, only a small fraction of the produced x-ray photons are used for imaging.
It is highly desirable to have an all-stationary computed tomography system. Such a system will reduce or eliminate the need to rotate the x-ray source around the patient. Furthermore, novel x-ray source geometries combined with the precise control of these x-ray sources can allow the development of imaging techniques and the refinement of current data acquisition methods.